It is known, for example, to locate a pressure relief valve between high and low pressure regions in a pneumatic or vacuum system such valves are actuated at a predetermined differential pressure to relieve the high pressure either to the low pressure region or to atmosphere. This known type of pressure relief valve uses either a spring or sometimes gravity alone to bias a valve stem towards a co-operating valve seat to maintain the valve in a normally closed position.
When this type of valve is required to operate free of oscillations it is common practice to incorporate a damping mechanism. The damping mechanism requires components which are manufactured to close tolerances and in environments where condensation and/or solid deposition may be formed within the valve this presents a risk of malfunction particularly with a spring-biased mechanism.